Rep DNA helicase is a DNA unwinding enzyme which together with helicase II is essential for DNA replication and repair in E. coli. Crystal structures of the E. coli Rep helicase in a binary complex with (ss) DNA and in a ternary complex with ss-DNA and ADP were determined to a resolution of 3.0 E and 3.2 E, respectively. The asymmetric unit in the crystals of the binary and ternary complexes contains two Rep monomers differing from each other by a reorientation of one of the domains, corresponding to a swiveling of 130 degree about a hinge region. This large motion results in the closing of a cleft involved in ss-DNA binding. Such domain movements, within the context of the Rep dimer, are sufficiently large to suggest that these may be coupled to translocation of the Rep dimer along DNA by a step-wise movement of the subunits. These structures represent the first view of a DNA helicase bound to DNA.